Considerable interest exists in the future use of carbon-graphite fibers due to their light weight and high strength. Presently, about 30 pounds of graphite are being utilized per plane in manufacture of several existing aircraft and future projected use is 1000 lbs/aircraft. Due to the need to reduce weight of automobiles to increase fuel efficiency, use in cars is also expected to increase dramatically over the next decade. The projected annual use of graphite fibers by 1990 is as follows:
TABLE 1 ______________________________________ Industry Pounds ______________________________________ Aerospace 10.sup.6 Automobile 10.sup.9 Consumer 10.sup.6 ______________________________________
Commercial carbon fiber is usually sold as a stranded material or as a woven cloth, having from 100 to 10,000 discrete thin fibers per strand. These fibers are prepared by heating a precursor such as rayon, pitch or polyacrylonitrile fiber to carbonize the fibers followed by a high temperature (2000.degree.-3000.degree. C.) graphitization treatment under stress in absence of oxygen during which it is believed that the carbon atoms rearrange into a hexagonal structure. The industry has developed fine strand multifilament products as the result of difficulties in manufacturing large diameter fiber of sufficiently high modulus. It will be noted that an extremely small fiber diameter is now the industry standard, and is not predicted to change very much in the immediate future:
______________________________________ Carbon Fiber Diameter 6.5 to 13 microns Modulus 50 million psi Fall Rate in air at 1 ATM about 2 cm/sec. Resistivity 1000 ohms/cm. Burnout 0.5 to 1.0 watt/cm. Contact Voltage Drop 2 to 5 volts ______________________________________
Recently a significant hazard has been recognized that could prevent the widespread use of graphite fibers. The fine fibers are conductive and are not oxidized nor vaporized at the temperatures experienced during a typical fire. During a fire the epoxy or other resin binder is destroyed at 400.degree.-600.degree. C. Fine graphite fibers and fragments are expelled from the composite, are entrained in the air and form aerosols. The aerosols can travel significant distances, invade or settle in unprotected electrical or electronic equipment and cause shorting, equipment failure, power failure and blackouts. Automobile fires are quite a common event and aircraft fires occur frequently. Such an event could cause disastrous consequences at or near airport, industrial or residential areas.
Since the surface temperature of combustion (fast oxidation) of graphite is in the vicinity of 1300.degree. C., fast oxidation of graphite is hardly reached by the simple combustion of a composite panel which occurs at typical surface temperature of 400-500 degrees C. Also, even if the requisite temperatures are reached, the rates of combustion (oxidation) are too low compared to the same rates for the resin. This has the practical implication that the resin burns away fast leaving behind the graphite fibers that do not combust in the absence of the supporting flame. The fiber diameter of 8 microns presents a 2500 cm.sup.2 surface area per gram of mass. This is very large and leads to rapid heat loss and is conducive to early extinction even if the combustion is initiated.
An additional property of the carbon fiber is the "red heat" behavior. It should be emphasized that in a shorting situation a single carbon fiber is most difficult to burn or consume. Rather the literature suggests that the carbon fiber becomes a glowing filament and does not pyrolyze or burn at least to about 2300.degree. K. And even above that temperature adequate air circulation is required to consume the fiber fully. A minimum of 16 grams of oxygen are needed to consume 12 grams of carbon, and hence in a closed area such as in the chassis of an electronic system, lack of air circulation and sufficiently high voltages may cause the fibers to develop a "red heat" condition and ignite adjacent flammable plastics and the like.
In order to permit such widespread use of graphite composites, the recognized electrical hazards must be overcome economically without sacrificing or compromising the proven good features (strength, weight and cost). This should preferably be achieved so that the fiber and composite are compatible with state of the art processing and equipment. Furthermore, modification of the fiber by coating or treatment must provide a good bond to the fiber and to the resin matrix.
One approach to solving this problem is disclosed in a prior patent application entitled Gasifiable Carbon-Graphite Fibers filed on Apr. 17, 1979 by Marshall F. Humphrey, Kumar N. R. Ramohalli and Warren L. Dowler under Ser. No. 030,836, the disclosure of which is expressly incorporated herein by reference.
This copending application discloses the coating of carbon-graphite fibers with a salt of a metal having a work function below 4.2 eV such as an alkaline earth metal salt. The coating catalytically enhances combustion of the fibers at temperatures below 1000.degree. C. such that the fibers self-support combustion and burn to produce a non-conductive ash even in the absence of an external flame. It is important that the coating of the metal salt be applied uniformly and economically to the graphite.
Ion plating and electrodeposition will provide a uniform coating of metal salt, but are costly and time consuming. Application of the coating by immersion in a solution of a salt followed by drying results in a non-uniform, clumpy coating, which was difficult to control, was not esthetically pleasing and caused non-uniform burning of the fibers.